With reference to FIG. 1, a ducted fan gas turbine engine generally indicated at 10 has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, and intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
A fan casing 24 surrounds the fan 12, and is typically joined at its rear end to a rear casing 25 at a point known as the A3 flange, and at its front end to the air intake casing 11 at a point known as the A1 flange. The fan casing 24 and the air intake 11 are typically joined together at a joint assembly which uses nuts and bolts, the shanks of the bolts arranged to pass axially through holes in annular radially extending flanges at the abutting ends of the fan casing 24 and the intake 11. A similar assembly typically joins the fan casing 24 to the rear casing 25.
As shown in FIG. 2, which is close-up view of a nut 26 and bolt pair connecting the annular flanges 27, 28 of an air intake 11 and a fan casing 24, it is customary to elongate the shank 30 of each bolt and provide a cylindrical collar 29 around the shank between the head 31 of the bolt and one of the flanges or between the nut 26 and one of the flanges. This increases the effective length of the shank and therefore the absolute axial extension of the shank prior to failure of the bolt. It is also possible for the cylindrical collar 29 to be a crushable collar such that at extreme loads the collars crushes to allow greater parting of the flanges 27, 28 prior to failure of the bolts.
During an extreme dynamic event, such as a fan-blade-off, the fan casing has initially to withstand the impact of the released fan blade. A few milliseconds later the compressor section surges and a pulse wave from the combustor passes through the fan casing. Following this, the out of balance rotor cause the other fan blades to rub and forces orbiting of the fan casing. The front joint assembly between the fan casing and the air intake has to reduce the transmission of orbiting forces to the intake while ensuring that the intake remains attached. The rear joint assembly between the rear casing and the fan casing has to withstand the surge pulse which urges the fan casing axially forward away from the rear casing and also expands the fan casing outwards due to the pressure in the pulse wave, but then must accommodate the forced orbiting of the fan casing. It is important that the bolts do not fracture and allow an “unzipping” of the flanges that would allow the intake to separate from the fan casing, or the fan casing to separate from the rear casing. It is also important that vibration transmitted to the aircraft is minimised. It is a regulatory certification requirement to perform a test in which a blade is deliberately released to prove that the engine is capable of accommodating such an unlikely event.
Focusing on the front joint assembly at the A1 flange, the crushing of the collars and extension of the bolt shanks can successfully absorb energy from the surge pulse. Further, the parted flanges of the joint assembly can accommodate forced orbiting of the fan casing 24, as the shanks, once the flanges are parted, are no longer forced to be parallel with the axes of the casings, i.e. they can rotate to an extent in the holes in the flanges thus reducing the forcing on the intake 11. However, particularly when the fan casing is formed of composite material, a joint assembly of the type shown in FIG. 2 can become impractical as large load spreaders and hole reinforcements are required to prevent damage to the composite, and these spreaders substantially increase the weight of the assembly.
Focusing on the rear joint assembly at the A3 flange, typically extendable bolts are used to absorb energy from the surge pulse whilst retaining the fan casing 24 coaxial with the rear casing 25. However, particularly when the fan casing 24 is formed of composite material, a joint assembly similar to the type shown in FIG. 2 can again become impractical as large load spreaders and hole reinforcements are required to prevent damage to the composite.
Thus there is a need for improved joint assemblies.